In the manufacture of semiconductor devices, metal contact lines and interconnections have been made primarily of aluminum heretofore. Aluminum can be processed by dry etching in a vacuum chamber at low temperatures to form volatile by-products which can be pumped away by the chamber exhaust system. Further, aluminum has low electrical resistance. However, as the dimensions of semiconductor devices become smaller, e.g., using 0.5 micron design rules, problems with the use of aluminum have become apparent. Aluminum has low mechanical strength, and thus when it becomes stressed, as by the application of current or because of thermal cycling, aluminum tends to crack, forming disconnects. The thinner the aluminum lines become, the more this problem is exacerbated.
The use of copper as an alternative to aluminum has been studied, because copper has higher mechanical strength, is more ductile than aluminum, and has excellent conductivity. However, copper has not been used for semiconductor device manufacture up till now, because it cannot be processed by conventional dry etching equipment and processes.
In order to make line connections on a substrate, after the deposition of a conductive metal layer over the substrate, a layer of photoresist is deposited and patterned to expose portions of the metal layer. The metal layer is then etched away in the exposed areas. Aluminum has been etched by reactive ion etching using a halogen compound or an organohalide, particularly a chloride, forming aluminum chloride for example as a by-product of the reaction. This reaction takes place at temperatures below about 500.degree. C. This low temperature etch is acceptable because it does not unduly damage a silicon wafer, or change diffused or ion implanted ions in or on the wafer used to form devices such as transistors.
However, copper does not form a corresponding copper chloride at low temperatures, but instead requires much higher temperatures of about 1000.degree. C. at normal pressures and temperatures of over about 500.degree. C. and up to about 600.degree. C. under vacuum. Such temperatures have adverse effects on underlying layers and state-of-the-art photoresists can not withstand such temperatures. Thus copper cannot be used as a metal connector using state of the art semiconductor processing.
However, because of the potential advantages of the use of copper lines as enumerated above, a method of patterning copper lines at low temperatures would be highly desirable.